Mutations in the hydrophobic core and in the protein–RNA interface
affect the packing and stability of icosahedral viruses
Sheila M. B. Lima
1
, David S. Peabody
2
, Jerson L. Silva
1
and Andre
´
a C. de Oliveira
1
1
Departamento de Bioquı
´
mica Me
´
dica, Instituto de Cie
ˆ
ncias Biome
´
dicas and Centro Nacional de Ressona
ˆ
ncia Magne
´
tica Nuclear de
Macromole
´
culas, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil;
2
Department of Molecular Genetics and

packing or the interaction at the protein–RNA interface,
result in changes in virus stability.
Keywords: hydrostatic pressure; MS2 bacteriophage; tem-
perature-sensitive mutants; urea; fluorescence.
The protein shells of viruses generally have several key
functions, including shielding of the nucleic acid, particle
maturation and conferring the ability to penetrate the host
cell and undergo disassembly. The coat proteins are usually
arranged in a shell with an icosahedral shape [1]. The
information required for successful assembly of a virus
particle is encoded in the native conformation of a capsid
protein subunit. Structural and thermodynamic approaches
have been employed to identify the general rules that govern
virus assembly [2–7].
The MS2 bacteriophage is an RNA virus of the
family Leviviridae, a group of single-stranded RNA
bacteriophages that infect F+ Escherichia coli cells. The
icosahedral shell of the MS2 virus particle has a
diameter of 260 A
˚
and is made up of 180 copies of
the coat protein subunit (M
r
13.7 · 10
3
)inaT¼3
surface lattice. Each virion also contains one copy of
the maturase protein, which is responsible for attach-
ment of the phage to E. coli F-pili. The coat protein
has two functions in the viral life cycle. First, it acts as

ria,
CEP 21941-590, Rio de Janeiro, RJ, Brazil.
Fax: + 55 21 2270 8647, Tel.: + 55 21 2562 6756,
E-mail:
Abbreviations: bis-ANS, bis-8-anilinonaphthalene-1-sulfonate; LB,
Luria–Bertani; p.f.u., plaque-forming units; ts, temperature sensitive;
WT, wild-type.
(Received 26 September 2003, accepted 7 November 2003)
Eur. J. Biochem. 271, 135–145 (2004) Ó FEBS 2003 doi:10.1046/j.1432-1033.2003.03911.x
High pressure is an efficient tool for studies on the
folding of proteins [15–18] and on the assembly of
supramolecular structures, such as viruses [6,7,19–23]. In
general, it has been found that individual capsid proteins
(monomers or dimers) are generally much less stable to
pressure than the assembled icosahedral particles [21,22].
The isolated capsid and the assembly intermediates assume
different partially folded states in the assembly pathway
[7]. Hydrostatic pressure permits controlled perturbation of
the subunit interactions and is a powerful tool for using to
study cavities in proteins [24,25]. The coat protein of
bacteriophage MS2 contains two tryptophan residues, thus
permitting the use of intrinsic fluorescence as a probe of
structural changes. Here, we study the stability against
pressure and urea of the MS2 bacteriophage and three
temperature sensitive (ts) mutants (Fig. 1B). Urea and
hydrostatic pressure are utilized to promote capsid disso-
ciation and denaturation, where the conformational chan-
ges are analyzed by fluorescence spectra, light scattering,
CD, HPLC, infectivity assays and the bis-8-anilinonaph-
thalene-1-sulfonate (bis-ANS) binding assay. We find that

buffer and purified by high-speed centrifugation
(35 000 r.p.m. for 14 h; SW41 rotor; Beckman) in a sucrose
gradient (10–50%). The sample concentration utilized in all
the experiments was 50 lgÆmL
)1
, except for CD experi-
ments (where the sample concentration was 100 lgÆmL
)1
).
Virus concentrations were determined by the method of
Bradford [26] using lysozyme as a standard. They were
confirmed by measuring the absorbance at 280 nm.
Spectroscopic measurements under pressure
Two important parts form the high pressure system: the
pressure generator and the high-pressure cell [27]. Fluores-
cence spectra and light scattering measurements were
recorded on an ISSK2 spectrofluorometer (ISS Inc.,
Champaign, IL, USA). Fluorescence spectra were quanti-
fied by evaluating the spectral center of mass, <m>, as
follows:
Fig. 1. Structure of the whole capsid of bacteriophage MS2 and of the
coat protein dimer bound to RNA. (A) The MS2 bacteriophage capsid is
colored according to the asymmetric units. (B) The coat protein of
MS2 bacteriophage bound to RNA, showing the location of the amino
acids (in space fill display) substituted in the temperature sensitive (ts)
mutants. The two polypeptide chains of the dimer are shown as blue
and green ribbons and the RNA molecule is shown in red (space fill).
Met88 is represented in brown (at the dimer surface), Thr45 in yellow
(interacting with RNA) and Asp11 in red, which appears to form a salt
bridge to Lys113 (cyan) on the alpha-helix of the adjacent subunit of

relation [15,16]:
ln½a
n
p
=ð1 À a
p
Þ ¼ pDV=RT þ ln½k
atm
=n
n
C
ðnÀ1Þ
ð3Þ
where k
atm
is the denaturation/dissociation constant at
atmospheric pressure, p corresponds to a given pressure, R
is the gas constant, T is the absolute temperature, n is the
number of subunits, and C is the protein concentration.
Each experiment was performed at least three times with
different protein preparations.
Light scattering
Light scattering measurements were made in an ISSK2
spectrofluorometer. Scattered light was collected at an angle
of 90° of the incident light. The samples were excited on
320 nm and collected in the same wavelength. This wave-
length was chosen because protein and RNA do not absorb
at 320 nm.
Chemical denaturation
The samples were incubated with increasing concentrations

)1
and the spectra were
obtained in 10 m
M
Tris, 30 m
M
NaCl (pH 7.5) buffer using
a 0.1 cm pathlength quartz cuvette. The spectropolarimeter
used was a Jasco J-715 1505 model.
Infectivity assays
An overnight culture of E. coli was diluted 1 : 20 (v/v) in
LB medium and cultured at 37 °C for 2 h in a rotary shaker.
Several phage dilutions, made in a standard buffer, were
plated in LB semisolid medium containing E. coli.The
plates were incubated overnight at 37 °C, after which the
MS2 and ts mutants were diluted and titered by quantifi-
cation of plaques resulting from phage-induced bacterial
lysis. The results are expressed as p.f.u. (plaque-forming
units) per mL.
Isolation of ts coat mutants
The MS2 coat protein gene was randomly mutagenized by
error-prone PCR [28] and introduced into pMS27 [29], a
plasmid from which infectious MS2 genomic RNA can be
produced by transcription from the T7 promoter. Transfec-
tion into strain CSH41(pAR1219) [30], which produces T7
RNA polymerase, and plating at 32 °C led to the production
of plaques, % 200 of which were picked to lawns of CSH41 on
duplicate plates. One plate was incubated at 32 °Candthe
other at 40 °C. After identification of mutants exhibiting a
growth defect at 40 °C, virus stocks were produced by

Peabody, unpublished results). We describe here some addi-
tional properties of three: D11N, T45S and M88V (Fig. 1B).
We used light scattering and intrinsic tryptophan fluor-
escence to monitor whole particle disassembly and subunit
denaturation, respectively. Intrinsic fluorescence of the coat
Ó FEBS 2003 Cavities and stability in MS2 capsid mutants (Eur. J. Biochem. 271) 137
protein in the absence of urea is blue shifted because the
tryptophan residues are buried in the hydrophobic interior
of the protein. As the protein unfolds, the tryptophan
residues become more exposed to the solvent and their
fluorescence maxima shift towards the red (Fig. 2). Figure 2
shows that the WT and mutant forms have different
susceptibilities at an intermediate urea concentration
(4.5
M
), but all fully dissociate and denature at a high urea
concentration (9.0
M
).
Trp32 clearly resides within the hydrophobic core of the
protein. The other, Trp82, is only partially solvent-exposed
[33]. Its environment is determined primarily by interactions
within the dimer, not by interactions between dimers. Thus,
tryptophan fluorescence should predominantly monitor
dimer denaturation rather than capsid dissociation. Mean-
while, light scattering measurements are sensitive to the size
of the particle and can be used to monitor capsid
dissociation. The WT virus and each mutant were subjected
to increasing concentrations of urea (1–9
M

utilized. Only for the D11N mutant was a small fraction
Fig. 2. Tryptophan fluorescence spectra of bacteriophage MS2. Spectra
of wild-type (WT) (circles), M88V (diamonds), D11N (squares) and
T45S (triangles) particles were recorded at atmospheric pressure in the
absence (filled symbols), or presence of 4.5
M
(unfilled symbols) or
9.0
M
(lines) urea. The excitation wavelength was 280 nm, and the
emission wavelength range was 300–420 nm. Standard buffer: 50 m
M
Tris/150 m
M
NaCl (pH 7.5). The sample concentration utilized was
50 lgÆmL
)1
.
Fig. 3. Dissociation and denaturation of wild-type (WT) and mutant MS2 particles. (A) Light scattering and spectral center of mass measurements of
WT MS2 as a function of urea concentration. To verify the dissociation and denaturation processes, we measured the light scattering of the particles
at 320 nm (d)andthespectralcenterofmassoftheparticles(s). (B) Urea-induced denaturation of MS2, as measured by tryptophan fluorescence
for: (d), WT MS2; (r), M88V; (j), D11N; and (m), T45S. (C) Urea-induced dissociation, as measured by light scattering for: (d), WT MS2; (r),
M88V; (j), D11N; and (m), T45S. For tryptophan fluorescence emission, the sample was excited at 280 nm and the emission was measured at 300–
420 nm. For the light scattering measurements, the sample was excited at 320 nm and the emission measured from 315 to 325 nm. Standard buffer:
50 m
M
Tris/150 m
M
NaCl (pH 7.5). Fluorescence data points are the average and standard deviation of three experiments (A and B) and light
scattering curves are representative of three measurements. The sample concentration utilized was 50 lgÆmL

The effects of high pressure on tryptophan fluorescence
emission spectra of the MS2 and ts mutants were also
investigated. Pressure produced complete dissociation of the
two mutants M88V and T45S. However, up to 3.4 kbar,
hydrostatic pressure was unable to promote complete
dissociation of WT and D11N particles, as measured by
fluorescence (Fig. 5A) and by light scattering (Fig. 5B). In
agreement with the urea studies described above, the D11N
mutant was more stable than the WT bacteriophage. All the
curves for pressure denaturation of WT and mutant
particles seem to have more than one transition, which
may indicate partially dissociated or denatured states.
However, because both fluorescence and light scattering
reveal the average properties, we cannot fully characterize
these potential intermediates.
The reversibility of the process was analyzed by deter-
mining the values of spectral center of mass (Fig. 5A,C),
which were measured after decompression and utilizing
HPLC (Fig. 4). Figure 5C shows that the spectra of the
sample subjected to compression and decompression are
similar to the non-treated sample, even in the case of the
M88V mutant. The elution of the samples after pressuriza-
tion in the same position as the native virus showed that the
particles were able to reassemble correctly, suggesting that
dissociation by pressure is at least partially reversible. To
further investigate the recovery, infectivity assays were
performed and when we used high pressure the phage titer
was similar to that of the control (Table 1). Thus, in spite of
the dissociation induced by pressure, the information for
correct reassembly seems to be largely preserved. In

Control
3.4 kbar
of pressure
4.5
M
urea
9.0
M
urea
MS2 10
8
10
8
10
5
ND
T45S 10
8
10
8
10
2
ND
M88V 10
8
10
7
10
2
ND

However, the WT bacteriophage and the D11N mutant
showed little change in structure, confirming their higher
stabilities. At higher urea concentrations, both WT bacterio-
phage and the D11N mutant lost the ellipticity at 218 nm,
indicating complete denaturation (results not shown).
Bis-ANS binding assay of MS2 and ts mutants
The fluorophore, bis-ANS, binds non-covalently to non-
polar segments in proteins, especially those in proximity to
positive charges [36]. Its binding is accompanied with a large
increase in its fluorescence quantum yield and it has been
Fig. 5. Pressure stability of bacteriophage MS2 and the temperature
sensitive (ts) mutants. The effect of pressure on the samples was ana-
lyzed at room temperature. (A) The effect was measured by the tryp-
tophan fluorescence emission of the spectral center of mass. The
samples were excited at 280 nm and the emission was measured from
300 to 420 nm for: (d), WT MS2; (r), M88V; j), D11N; and (m),
T45S. (B) Light scattering measurements of MS2 and ts mutants under
pressure. (d) MS2 bacteriophage and the ts mutants (r)M88V,(j)
D11N, and (m) T45S. The excitation wavelength was 320 nm and the
emission wavelength range was 315–325 nm. The incubation time at
each pressure was 10 min. Other conditions were as described in the
legend to Fig. 2. The unfilled symbols correspond to the respective
values after pressure release. Fluorescence data points are the average
and SD of three experiments, and light scattering curves are the rep-
resentative of three measurements. (C) Fluorescence emission spectra
of M88V mutant particles before (unbroken line), under 3.4 kbar of
pressure (broken line, or after decompression (dotted lines).
Table 2. [U]
½
and p

DV/n.
140 S. M. B. Lima et al. (Eur. J. Biochem. 271) Ó FEBS 2003
used to probe protein structural changes [37,38]. At
atmospheric pressure and in the absence of urea, the MS2
bacteriophage and D11N mutant did not bind bis-ANS
(Fig. 7B), showing that these particles do not present
exposed hydrophobic segments. High urea concentrations
did not promote significant binding of bis-ANS to any of
the particles, with the exception of T45S, which bound a
small amount of bis-ANS, increasing, by twofold, the
emission of the probe (Fig. 7B). M88V and WT particles
treated with pressure did not show significant changes in
bis-ANS binding. However, when the T45S mutant was
denatured and dissociated by pressure, a sixfold increase in
the emission of the probe occurred (Fig. 7A), suggesting
that the conformation of the pressure-denatured state is
different from that of the urea-denatured state.
Discussion
In the last 20 years, the structure of many viruses has been
solved by X-ray crystallography [1]. However, despite
extensive knowledge of their structures, the mechanisms of
virus assembly and disassembly are still poorly understood.
In viruses, interactions within and between capsid subunits
must be strong enough to ensure virus particle stability and
protection of the genome, but weak enough to permit
uncoating or release of the genome upon interaction with
the cell. The coat protein of the bacteriophage MS2 has two
basic functions, (a) specific RNA binding for translational
repression and genome encapsidation and (b) formation of
the capsid structure. Structural and genetic analysis permit

bacteriophage [14] and the M88V mutant (by substitution
using the same program) for the existence of internal cavities
in this region in the structure. A significant cavity in the WT
phage structure in the neighboring Met88 residue was
identified. After substitution of this residue with valine, the
cavity volume increased to 43 A
˚
3
, reflecting a reduction in
the surface area of the residue (% 59 A
˚
2
)andinthe
interactions occurring there (Fig. 8). Met88 resides in the
Fig. 6. UV CD spectra of wild-type MS2
bacteriophage and the temperature sensitive (ts)
mutants. Conformational changes in the sec-
ondary structure of bacteriophage MS2 and ts
mutants were analyzed in the presence of
4.5
M
urea (hollow symbols). Filled symbols
correspond to the samples in the absence of
urea. Wavelength range: 300–210 nm. The
samples of MS2 bacteriophage and ts mutants
were diluted to a final concentration of
100 lgÆmL
)1
and the spectra were measured in
buffer (10 m

the creation of a cavity, the substitution of Val for Met
might produce steric clashes as a result of introduction of
the beta branched side-chain where the unbranched Met
side-chain is ordinarily packed.
The physical basis for the reduced stability of the T45S
mutant may have a similar explanation. Although residue
45 resides on the surface of the protein and makes no
obvious stabilizing interactions with other amino acid side-
chains, this part of the protein interacts with genomic RNA
[33,41,47]. Such interactions probably contribute to the
stability of the virus particle (Fig. 1B). The crystal structure
of coat protein in complex with the translational operator
shows interaction between Thr45 and RNA [41]. The X-ray
structure of the virus particle itself shows significant electron
density in the vicinity of Thr45, indicating that many
individual subunits apparently contact RNA at this posi-
tion, albeit in a presumably non-specific manner [13,14].
Thus, it is clear that the T45S substitution destabilizes the
capsid because of a perturbation of the protein–RNA
interaction, as assessed by the several criteria reported here.
The high sensitivity of coat protein folding/stability to
Thr45 substitution was previously inferred from electroph-
oretic studies [41]. Nineteen substitutions were introduced in
position 45 and none of the amino acids was a completely
acceptable replacement for threonine. Every mutant showed
loss of translational repression, increased insolubility and/or
degradation, and failure to produce normal quantities of
virus-like particles. However, the T45S mutant was the most
affected in the translational repressor activity [41].
The T45S mutant was more sensitive to urea than to high

mutant. (B) bis-ANS binding to MS2 bacteriophage and mutants
under conditions of increasing urea concentrations. Structural changes
were also analyzed by a fluorescent probe (bis-ANS) emission, at a
final concentration of 2 l
M
(d) MS2 bacteriophage and the ts mutants
(m)T45Sand(j) D11N. The excitation wavelength was 360 nm and
the emission wavelength range was 400–600 nm. The data shown are
representative of three experiments.
142 S. M. B. Lima et al. (Eur. J. Biochem. 271) Ó FEBS 2003
(corresponding to the b-sheet) is smaller. An appealing
interpretation for these findings is that the lower stability of
the coat protein shell is counterbalanced by a more
structured RNA, which results in a similar infectivity.
The increased stability of the D11N mutant presents a
puzzle. Like the others, this mutant was isolated for its ts
growth phenotype, implying that it possesses decreased
thermal stability. Why then is the virus apparently more
stable? The X-ray structure of MS2 shows that Asp11
participates in both intra- and interchain interactions. It
appears to form a salt bridge to Lys113 on the alpha-helix of
the adjacent subunit of the dimer, and may thus help to
stabilize the dimer interface (residues in red and blue colors
in Fig. 1B). But Asp11 makes another interaction. It resides
in b-strand A, just before the turn that connects it to bB.
Here it seems to position itself to the H-bond, through a
carbonyl oxygen of its side-chain, to the main chain amide
of Gly13 within the turn, thus possibly exerting a stabilizing
influence on the turn. The D11N substitution might be
expected to have a destabilizing effect on both interactions,

increase in the cavity. Interaction with RNA is also sensitive,
and its perturbation (in the case of T45S replacement) also
leads to lower stability. On the other hand, capsid proteins
cannot be highly packed otherwise they would lose the
flexibility needed for virus assembly. In this context, the
intricate interactions between capsid protein and RNA are
Fig. 8. Cavity increase occuring in the M88V mutation. Using the program
VMD
and a probe radius of 1.4 A
˚
, we analyzed the pdb coordinates of
wild-type (WT) bacteriophage and the M88V mutant for the existence of internal cavities in this region. The figure shows the region on coat protein
in the asymmetric unit of the capsid around residue 88. The methionine residue is shown in yellow and the valine is represented in green. A
significant cavity was identified in the WT phage structure (A and B) in the neighboring region of the Met88 residue. After the substitution of this
residue with valine (C and D), the cavity volume increased to 43 A
˚
3
, reflecting a reduction in the surface area of the residue (% 59 A
˚
2
)andinthe
interactions occurring there.
Ó FEBS 2003 Cavities and stability in MS2 capsid mutants (Eur. J. Biochem. 271) 143
crucial for assembling the whole particle. It is also interest-
ing that the mutants had a ts phenotype, which means a
high sensitivity to both high and low temperatures. The
presence of cavities that confer the decreased stability to
pressure is also usually related to both decreased stability to
low and high temperatures [15,17,18].
Acknowledgements

o
Universita
´
ria Jose
´
Bonifa
´
cio(FUJB)ofBraziltoJ.L.S.andA.C.O.,
and by a grant from the National Institutes of Health (NIH) to D.S.P.
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